The invention relates to a radiation-sensitive resist composition, as well as to a method for manufacturing highly resolved relief structures.
In semiconductor production, highly resolving, radiation-sensitive resists are needed to produce fine patterns. In lithographic processes, a substrate is covered with a thin layer of this type of resist, and the desired pattern is transferred, initially as a latent image, into this layer--through projection exposure or beam control. If indicated, following further treatment steps, such as baking and silylation, the latent image is then developed, a relief pattern that serves as a mask for subsequent substrate etching processes being formed in the resist layer. The substrate can thereby consist of a semiconductor material such as silicon, or it can be a polymeric layer of organic material, i.e., one then has a so-called two-layer resist (c.f., e.g., L. F. Thompson, C. G. Willson, M. J. Bowden "Introduction to Microlithography", ACS Symposium Series 219, American Chemical Society, Washington 1983, pp. 16, 20 and 21; W. M. Moreau "Semiconductor Lithography", Plenum Press, New York 1988, page 4).
Besides a film-forming base polymer, a radiation-active compound constitutes the main component of the resist or of the resist layer. The radiation-active component, which is generally a diazo compound or a so-called crivello salt, is converted into an acid through absorption of incident radiation, such as visible light, near ultraviolet light (NUV), deep ultraviolet light (DUV), X-ray radiation, electron-beam radiation and ion-beam radiation. In the case of positively working, wet-developable resists, in some instances after a baking process, this acid increases the solubility of the irradiated regions in a developer (c.f.: L. F. Thompson et al., loc. cit., pp. 88-90 and 113-115). On the other hand, in the case of negatively working, wet-developable resists, for example, so-called "image reversal" resists, an acid-catalyzed cross-linking takes place in the irradiated regions, through which means the solubility in the developer is reduced (c.f.: "Proc. SPIE", vol. 1086 (1989), pp. 117 128).
When working with dry-developable resist systems, a silylation of the resist layer is carried out at the surface using suitable gaseous or liquid silicon-containing agents, and, in fact, when working with positively working resists, selectively in the unexposed regions and, when working with negatively working resists, selectively in the exposed regions. The development takes place in this case in an anisotropic, oxygen-containing plasma, the silylated regions serving at the surface of the resist layer as a resistant etching mask (c.f.: "Encycl. Polym. Sci. Eng.", vol. 9 (1987), p. 132). A high resolution capability is achieved both when working with wet-developable as well as with dryodevelopable resist systems through a greatest possible difference in the acid concentration between the exposed and the unexposed regions.
Because of diffraction phenomena, lens defects, etc., an image of a given object pattern (=mask pattern) having a modulation of unexposed regions.
Because of diffraction phenomena, lens defects, etc., an image of a given object pattern (=mask pattern) having a modulation of M.sub.object =1 is only able to be formed in the image plane with a modulation of M.sub.image &lt;1, i.e., the value for the optical modulation-transfer function (MTF.sub.optic =M.sub.image /M.sub.object) lies clearly under the maximum value of 1, particularly for fine patterns to be imaged (c.f.: L. F. Thompson et al., loc. cit., p. 36; W. M. Moreau, loc. cit., p. 357).
The result is that with an increasingly finer structure dimension, the image of the mask pattern formed in the image plane (=resist plane) becomes less and less sharp. For this reason, the resist layer is, in principle, also always exposed somewhat at those sites that actually should remain unexposed. Therefore, with an increasingly finer structure dimension, the difference in the acid concentration between those regions of the resist layer that are to be irradiated and those that are not to be irradiated becomes less and less. The limit of the resist resolution is then reached when the difference in concentration is too small for there to be a difference in solubility characteristics (or also silylation characteristics) of these two regions, i.e., when--for a given mask pattern--the value for the so-called critical modulation-transfer function of the resist
CMTF.sub.resist =(10.sup.1/.gamma. -1)/(10.sup.1/.gamma. +1) (.gamma.=contrast) exceeds the value of the optical modulation-transfer function (MTF.sub.optic &lt;CMTF.sub.resist). Therefore, a high contrast is indispensable for a high resolution capability of the resist (c.f., also: W. M. Moteau, loc. cit., pp. 368-371).
Now, the problem is that when conventional resist compositions are used, for example on the basis of novolak polymers and diazo naphthoquinones, or of t-boc-protected polymers and crivello salts, one cannot prevent acid from forming undesirably in those regions which are supposed to remain unexposed per se. The resultant reduced contrast of the resist increases the value for the CMTF.sub.resist and, thus, limits its resolution capability for fine patterns.